linux-kernel - Re: [PATCH v2] rust: macros: fix soundness issue in `module!` macro

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Message-ID: <bcbc1c4c-1e26-4afc-86ad-5cefa3dd26e0@proton.me>
Date: Tue, 02 Apr 2024 12:47:34 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, Martin Rodriguez Reboredo <yakoyoku@...il.com>, Asahi Lina <lina@...hilina.net>, Sumera Priyadarsini <sylphrenadin@...il.com>, Neal Gompa <neal@...pa.dev>, Thomas Bertschinger <tahbertschinger@...il.com>, Andrea Righi <andrea.righi@...onical.com>, Matthew Bakhtiari <dev@...k.me>, Adam Bratschi-Kaye <ark.email@...il.com>, stable@...r.kernel.org, Masahiro Yamada <masahiroy@...nel.org>, Wedson Almeida Filho <wedsonaf@...gle.com>, Finn Behrens <me@...enk.dev>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: macros: fix soundness issue in `module!` macro

On 02.04.24 00:17, Boqun Feng wrote:
> On Mon, Apr 01, 2024 at 10:01:34PM +0000, Benno Lossin wrote:
>> On 01.04.24 23:10, Boqun Feng wrote:
>>> On Mon, Apr 01, 2024 at 06:52:50PM +0000, Benno Lossin wrote:
>>> [...]
>>>> +            // Double nested modules, since then nobody can access the public items inside.
>>>> +            mod __module_init {{
>>>> +                mod __module_init {{
>>>> +                    use super::super::{type_};
>>>> +
>>>> +                    /// The \"Rust loadable module\" mark.
>>>> +                    //
>>>> +                    // This may be best done another way later on, e.g. as a new modinfo
>>>> +                    // key or a new section. For the moment, keep it simple.
>>>> +                    #[cfg(MODULE)]
>>>> +                    #[doc(hidden)]
>>>> +                    #[used]
>>>> +                    static __IS_RUST_MODULE: () = ();
>>>> +
>>>> +                    static mut __MOD: Option<{type_}> = None;
>>>> +
>>>> +                    // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
>>>> +                    // freed until the module is unloaded.
>>>> +                    #[cfg(MODULE)]
>>>> +                    static THIS_MODULE: kernel::ThisModule = unsafe {{
>>>> +                        kernel::ThisModule::from_ptr(&kernel::bindings::__this_module as *const _ as *mut _)
>>>
>>> While we're at it, probably we want the following as well? I.e. using
>>> `Opaque` and extern block, because __this_module is certainly something
>>> interior mutable and !Unpin.
>>>
>>> diff --git a/rust/macros/module.rs b/rust/macros/module.rs
>>> index 293beca0a583..8aa4eed6578c 100644
>>> --- a/rust/macros/module.rs
>>> +++ b/rust/macros/module.rs
>>> @@ -219,7 +219,11 @@ mod __module_init {{
>>>                        // freed until the module is unloaded.
>>>                        #[cfg(MODULE)]
>>>                        static THIS_MODULE: kernel::ThisModule = unsafe {{
>>> -                        kernel::ThisModule::from_ptr(&kernel::bindings::__this_module as *const _ as *mut _)
>>> +                        extern \"C\" {{
>>> +                            static __this_module: kernel::types::Opaque<kernel::bindings::module>;
>>> +                        }}
>>> +
>>> +                        kernel::ThisModule::from_ptr(__this_module.get())
>>>                        }};
>>>                        #[cfg(not(MODULE))]
>>>                        static THIS_MODULE: kernel::ThisModule = unsafe {{
>>>
>>> Thoughts?
>>
>> I am not sure we need it. Bindgen generates
>>
>>       extern "C" {
>>           pub static mut __this_module: module;
>>       }
>>
>> And the `mut` should take care of the "it might be modified by other
>> threads".
> 
> Hmm.. but there could a C thread modifies some field of __this_module
> while Rust code uses it, e.g. struct module has a list_head in it, which
> could be used by C code to put another module next to it.

This still should not be a problem, since we never actually read or
write to the mutable static. The only thing we are doing is taking its
address. `addr_of_mut!` should be sufficient. (AFAIK `static mut` is
designed such that it can be mutated at any time by any thread. Maybe
Gary knows more?)

-- 
Cheers,
Benno